1. Field of the Invention
This invention relates to a finish composition for carbon fibers, and is particularly directed to an epoxy resin finish composition for carbon fibers, containing a non-ionic emulsifier, and to a method of treating the surface of carbon fibers with an aqueous emulsion of an epoxy resin.
2. History of the Prior Art
Carbon fiber is conventionally produced by subjecting an organic polymer fiber to various conditions of temperature and atmosphere. Thus, for example, polyacrylonitrile fiber may be heated at a temperature in the range of 200.degree. to 300.degree. C. in an oxidizing atmosphere and subsequently heated at a temperature of at least about 1000.degree. C. in an inert atmosphere to produce carbon fiber.
It is known to provide carbon fibers with a sizing to facilitate handling and processing. When such fibers are processed without application of a sizing, they tend to fuzz and may actually separate or break as they are pulled over pulleys, rollers, etc.
In many applications, carbon fibers, filaments, yarns and tows are woven into a fabric or tape or otherwise interlaced or overlapped such that the strands are in contact with other similar strands. Because of the high moduli, brittleness and relatively poor abrasion resistance of the strands or the fibers which comprise them, they are difficult to handle during their processing and weaving into fabrics and tapes. These problems have been overcome in the past by the application to the fibers of a size.
Thus, the purpose of applying a sizing or a finish is to coat the carbon fibers or tows to prevent damage thereto. The possibility of damage to the carbon fibers or tows occurs during pre-pregging when the fibers are pulled over pulleys and other obstacles in the processing equipment. Occasionally, carbon fibers will break at an eyelet and hang up. This can create serious problems in the processing, since even if only two percent filament breakage occurs, this is generally considered unacceptable.
U.S. Pat. No. 4,409,288 to Raymond G. Spain discloses an improved aqueous sizing emulsion for treating carbon fibers comprising an epoxy resin and an effective amount of a two component emulsifying system, one of the emulsifying components being a long chain aliphatic alcohol containing from 8 to 18 carbon atoms and the other emulsifying component being a quaternary ammonium salt. The use of such emulsifying agents which are present on the coated fibers after drying does not degrade the properties of the composites ultimately made from the carbon fibers so sized.
However, the dried finish on the carbon fibers produced employing the aqueous emulsion of the above patent tends to stiffen, and such stiffening generally increases at higher temperatures. For example, it is not uncommon to transport the finished carbon fibers on tightly wound bobbins in a railroad boxcar where the inside temperature may reach 140.degree. F., or to store such materials in a warehouse under similar temperature conditions.
Accordingly, it would be desirable to produce a stable water based epoxy emulsion composition or finish for carbon fibers which protects the carbon fiber on handling, utilizing an emulsifier which improves the properties of the carbon fibers.
It would furthermore be desirable to provide a stable non-coagulating epoxy resin emulsion of the above type, for coating carbon fibers to produce a finish on such fibers which does not cause substantial stiffening of the carbon fibers containing the finish on moderate heating of such carbon fibers.
It would still furthermore be desirable to provide a process for treating carbon fibers with an epoxy resin emulsion of the above type to produce an improved finish on the carbon fibers.